Sweden has always been a relatively calm and safe country; the only concern for tourists has been long lines at the new ABBA museum. But that all changed with the recent sighting of a pacu, a fish that’s closely related to the piranha.

While the waters of Øresund, the strait that separates Denmark and Sweden, are normally still and relaxing, the sighting has prompted the nearby Natural History Museum of Denmark to release a serious warning: “Keep your swimwear on if you’re bathing in the Sound these days — maybe there are more out there!”

The fish is more often found in warmer climates — in fact it’s the first time that it has been reported in Scandinavian waters — and while it’s not lethal, it doesn’t have a good reputation. Henrik Carl of the Danish museum pointed out that, “the pacu is not normally dangerous to people but it has quite a serious bite, there have been incidents in other countries, such as Papua New Guinea, where some men have had their testicles bitten off.”

The pacu can grow to 25 kilograms (about 55 pounds), and in large parts of the USA and Asia it’s considered an invasive species. So how did it end up in Scandinavian waters? It is thought that the fish may have escaped from a nearby aquarium.